[EE] Resistor temperature rise

[Bob Blick]

What techniques are most effective in keeping power resistors from desoldering themselves from PC boards? I'm thinking there are basically three methods:

1. Don't use them at anywhere near their rated power. That's fine except if you need to tolerate the occasional overload condition.

2. Pour extra copper on and around the pads.

3. Use as much length of the wire leads as possible. That could really decrease the rigidity of the mounting.

4. ??

Attached is a picture of the temperature chart. I'm using the 5 watt AC05. Note the 260 degree K rise at rated power.

If I fold or clinch the leads it will keep the resistor from falling out, but the solder is going to melt. I imagine that won't give the greatest electrical connection :)

Datasheet here:

http://www.vishay.com/docs/28730/acac-at.pdf

Thanks,

Bob

[Richard Prosser]

The old-school way was as shown below - bend the leads around in a loop and
space the resistor off the pcb to permit airflow. But not very vibration
friendly! Adding a topside solder fillet may make the joint more rugged

Or use a larger resistor that can be properly clamped & heatsunk.

RP

On 17 January 2018 at 08:15, Bob Blick <***@outlook.com> wrote:

> What techniques are most effective in keeping power resistors from
> desoldering themselves from PC boards? I'm thinking there are basically
> three methods:
>
> 1. Don't use them at anywhere near their rated power. That's fine except
> if you need to tolerate the occasional overload condition.
>
> 2. Pour extra copper on and around the pads.
>
> 3. Use as much length of the wire leads as possible. That could really
> decrease the rigidity of the mounting.
>
> 4. ??
>
> Attached is a picture of the temperature chart. I'm using the 5 watt AC05.
> Note the 260 degree K rise at rated power.
>
> If I fold or clinch the leads it will keep the resistor from falling out,
> but the solder is going to melt. I imagine that won't give the greatest
> electrical connection :)
>
> Datasheet here:
>
> http://www.vishay.com/docs/28730/acac-at.pdf
>
> Thanks,
>
> Bob
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> http://mailman.mit.edu/mailman/listinfo/piclist
>
>

[Mark Rages]

Power resistors are often spaced off the board with two short pieces
of spaghetti tubing.

If you use metal-housed resistors you can bolt them to a heatsink.

Regards,
Mark

On Tue, Jan 16, 2018 at 12:48 PM, Richard Prosser <***@gmail.com> wrote:
> The old-school way was as shown below - bend the leads around in a loop and
> space the resistor off the pcb to permit airflow. But not very vibration
> friendly! Adding a topside solder fillet may make the joint more rugged
>
> Or use a larger resistor that can be properly clamped & heatsunk.
>
> RP
>
> On 17 January 2018 at 08:15, Bob Blick <***@outlook.com> wrote:
>
>> What techniques are most effective in keeping power resistors from
>> desoldering themselves from PC boards? I'm thinking there are basically
>> three methods:
>>
>> 1. Don't use them at anywhere near their rated power. That's fine except
>> if you need to tolerate the occasional overload condition.
>>
>> 2. Pour extra copper on and around the pads.
>>
>> 3. Use as much length of the wire leads as possible. That could really
>> decrease the rigidity of the mounting.
>>
>> 4. ??
>>
>> Attached is a picture of the temperature chart. I'm using the 5 watt AC05.
>> Note the 260 degree K rise at rated power.
>>
>> If I fold or clinch the leads it will keep the resistor from falling out,
>> but the solder is going to melt. I imagine that won't give the greatest
>> electrical connection :)
>>
>> Datasheet here:
>>
>> http://www.vishay.com/docs/28730/acac-at.pdf
>>
>> Thanks,
>>
>> Bob
>> --
>> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
>> View/change your membership options at
>> http://mailman.mit.edu/mailman/listinfo/piclist
>>
>>
>
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>



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[John Gardner]

Heat-sink the resistor?

...


On 1/16/18, Bob Blick <***@outlook.com> wrote:
> What techniques are most effective in keeping power resistors from
> desoldering themselves from PC boards? I'm thinking there are basically
> three methods:
>
> 1. Don't use them at anywhere near their rated power. That's fine except if
> you need to tolerate the occasional overload condition.
>
> 2. Pour extra copper on and around the pads.
>
> 3. Use as much length of the wire leads as possible. That could really
> decrease the rigidity of the mounting.
>
> 4. ??
>
> Attached is a picture of the temperature chart. I'm using the 5 watt AC05.
> Note the 260 degree K rise at rated power.
>
> If I fold or clinch the leads it will keep the resistor from falling out,
> but the solder is going to melt. I imagine that won't give the greatest
> electrical connection :)
>
> Datasheet here:
>
> http://www.vishay.com/docs/28730/acac-at.pdf
>
> Thanks,
>
> Bob
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[Van Horn, David]

I take it water cooling is out.

No real magic here bud, you gotta let the heat flow one way or the other, or the temperature will rise till it does.


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[Bob Blick]

So basically they sell 5 watt resistors but if you solder them you can't push more than 2.5 watts out of them? I've looked at a lot of datasheets lately, and none of them make any mention at all about connection or mounting methods.

Bob


________________________________________
From: piclist-***@mit.edu <piclist-***@mit.edu> on behalf of Van Horn, David
Sent: Tuesday, January 16, 2018 12:34 PM
To: Microcontroller discussion list - Public.
Subject: RE: [EE] Resistor temperature rise


I take it water cooling is out.

No real magic here bud, you gotta let the heat flow one way or the other, or the temperature will rise till it does.



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[Van Horn, David]

Creative specmanship. The resistors themselves are rated at 5W.
They either have materials that will tolerate the temperature, or somewhere in a footnote on page 97 of the spec, give you a spec for thermal resistance to environment or some such.



-----Original Message-----
From: piclist-***@mit.edu [mailto:piclist-***@mit.edu] On Behalf Of Bob Blick
Sent: Tuesday, January 16, 2018 2:18 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] Resistor temperature rise

So basically they sell 5 watt resistors but if you solder them you can't push more than 2.5 watts out of them? I've looked at a lot of datasheets lately, and none of them make any mention at all about connection or mounting methods.

Bob


________________________________________
From: piclist-***@mit.edu <piclist-***@mit.edu> on behalf of Van Horn, David
Sent: Tuesday, January 16, 2018 12:34 PM
To: Microcontroller discussion list - Public.
Subject: RE: [EE] Resistor temperature rise


I take it water cooling is out.

No real magic here bud, you gotta let the heat flow one way or the other, or the temperature will rise till it does.



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[Sean Breheny]

As you know, Bob, many component vendors do practice fuzzy specsmanship.
However, I think resistors are rather mild in this case. For example, it is
standard practice for MOSFET vendors to rate their FETs according to the
amount of current they could carry if you somehow magically kept the case
at 25 deg C.This means that they only have to account for the junction to
case thermal impedance as well as the heating in the leads of the package.
In real life you will NEVER get away with that much current for more than a
split second.

As I understand it, for through-hole resistors, the rating is the max power
the resistor can dissipate continuously, in free air (i.e., not mounted to
a PCB) with the surrounding air at 25 deg C, for some specified lifetime,
without changing its resistance value by more than X percent.

A good and generally accepted rule of thumb is that you can use a resistor
at up to half its rated power, continuously, soldered to a PCB, provided
that there are no other significant sources of heat in the vicinity of the
resistor AND the air temperature in the enclosure (if there is one) is not
much above 25 C even considering the dissipation from the resistor. This
includes the PCB traces leading to the resistor - they must be large enough
that they have negligible heating from the current passing through them. I
had a board once where the (low value current sense) resistors lifted the
traces off the board because I failed to take into account that the traces
and the resistors would both generate heat. In other words, the resistor
alone was within its specs and the traces were sized properly according to
PCB trace width calculators but both of those ratings assume that their
contribution is by far dominant but in this case, the heat from the
resistor heated the copper just enough that it went into thermal runaway at
the continuous operating current.



On Tue, Jan 16, 2018 at 4:17 PM, Bob Blick <***@outlook.com> wrote:

> So basically they sell 5 watt resistors but if you solder them you can't
> push more than 2.5 watts out of them? I've looked at a lot of datasheets
> lately, and none of them make any mention at all about connection or
> mounting methods.
>
> Bob
>
>
> ________________________________________
> From: piclist-***@mit.edu <piclist-***@mit.edu> on behalf of Van
> Horn, David
> Sent: Tuesday, January 16, 2018 12:34 PM
> To: Microcontroller discussion list - Public.
> Subject: RE: [EE] Resistor temperature rise
>
>
> I take it water cooling is out.
>
> No real magic here bud, you gotta let the heat flow one way or the other,
> or the temperature will rise till it does.
>
>
>
> --
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> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>
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[Bob Blick]

A funny thing, though, is that I originally was using 2 watt resistors(metal oxide and also a different brand) and got these 5 watt wirewound ones hoping for a little cooler running. But comparing the graphs in their datasheets I noticed that, at 2 watts, the 5 watt and 2 watt resistors both showed very similar temperature rise. I would have thought the larger size of the 5 watt resistors would have had some effect. I have not measured the actual temperature rise though, who knows. Obviously the wirewound resistors can withstand a higher operating temperature than the metal oxide ones. Long term operation is not high power, I just wanted a little extra headroom for temporary overloads.

Cheerful regards,

Bob

________________________________________
From: piclist-***@mit.edu <piclist-***@mit.edu> on behalf of Sean Breheny
Sent: Tuesday, January 16, 2018 4:08 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] Resistor temperature rise

As you know, Bob, many component vendors do practice fuzzy specsmanship.
However, I think resistors are rather mild in this case. For example, it is
standard practice for MOSFET vendors to rate their FETs according to the
amount of current they could carry if you somehow magically kept the case
at 25 deg C.This means that they only have to account for the junction to
case thermal impedance as well as the heating in the leads of the package.
In real life you will NEVER get away with that much current for more than a
split second.

As I understand it, for through-hole resistors, the rating is the max power
the resistor can dissipate continuously, in free air (i.e., not mounted to
a PCB) with the surrounding air at 25 deg C, for some specified lifetime,
without changing its resistance value by more than X percent.

A good and generally accepted rule of thumb is that you can use a resistor
at up to half its rated power, continuously, soldered to a PCB, provided
that there are no other significant sources of heat in the vicinity of the
resistor AND the air temperature in the enclosure (if there is one) is not
much above 25 C even considering the dissipation from the resistor. This
includes the PCB traces leading to the resistor - they must be large enough
that they have negligible heating from the current passing through them. I
had a board once where the (low value current sense) resistors lifted the
traces off the board because I failed to take into account that the traces
and the resistors would both generate heat. In other words, the resistor
alone was within its specs and the traces were sized properly according to
PCB trace width calculators but both of those ratings assume that their
contribution is by far dominant but in this case, the heat from the
resistor heated the copper just enough that it went into thermal runaway at
the continuous operating current.



On Tue, Jan 16, 2018 at 4:17 PM, Bob Blick wrote:

> So basically they sell 5 watt resistors but if you solder them you can't
> push more than 2.5 watts out of them? I've looked at a lot of datasheets
> lately, and none of them make any mention at all about connection or
> mounting methods.
>
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[John Gardner]

I guess there's a reason data sheets are (usually) anonymous?

...


On 1/17/18, Bob Blick <***@outlook.com> wrote:
> A funny thing, though, is that I originally was using 2 watt resistors(metal
> oxide and also a different brand) and got these 5 watt wirewound ones hoping
> for a little cooler running. But comparing the graphs in their datasheets I
> noticed that, at 2 watts, the 5 watt and 2 watt resistors both showed very
> similar temperature rise. I would have thought the larger size of the 5 watt
> resistors would have had some effect. I have not measured the actual
> temperature rise though, who knows. Obviously the wirewound resistors can
> withstand a higher operating temperature than the metal oxide ones. Long
> term operation is not high power, I just wanted a little extra headroom for
> temporary overloads.
>
> Cheerful regards,
>
> Bob
>
> ________________________________________
> From: piclist-***@mit.edu <piclist-***@mit.edu> on behalf of Sean
> Breheny
> Sent: Tuesday, January 16, 2018 4:08 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] Resistor temperature rise
>
> As you know, Bob, many component vendors do practice fuzzy specsmanship.
> However, I think resistors are rather mild in this case. For example, it is
> standard practice for MOSFET vendors to rate their FETs according to the
> amount of current they could carry if you somehow magically kept the case
> at 25 deg C.This means that they only have to account for the junction to
> case thermal impedance as well as the heating in the leads of the package.
> In real life you will NEVER get away with that much current for more than a
> split second.
>
> As I understand it, for through-hole resistors, the rating is the max power
> the resistor can dissipate continuously, in free air (i.e., not mounted to
> a PCB) with the surrounding air at 25 deg C, for some specified lifetime,
> without changing its resistance value by more than X percent.
>
> A good and generally accepted rule of thumb is that you can use a resistor
> at up to half its rated power, continuously, soldered to a PCB, provided
> that there are no other significant sources of heat in the vicinity of the
> resistor AND the air temperature in the enclosure (if there is one) is not
> much above 25 C even considering the dissipation from the resistor. This
> includes the PCB traces leading to the resistor - they must be large enough
> that they have negligible heating from the current passing through them. I
> had a board once where the (low value current sense) resistors lifted the
> traces off the board because I failed to take into account that the traces
> and the resistors would both generate heat. In other words, the resistor
> alone was within its specs and the traces were sized properly according to
> PCB trace width calculators but both of those ratings assume that their
> contribution is by far dominant but in this case, the heat from the
> resistor heated the copper just enough that it went into thermal runaway at
> the continuous operating current.
>
>
>
> On Tue, Jan 16, 2018 at 4:17 PM, Bob Blick wrote:
>
>> So basically they sell 5 watt resistors but if you solder them you can't
>> push more than 2.5 watts out of them? I've looked at a lot of datasheets
>> lately, and none of them make any mention at all about connection or
>> mounting methods.
>>
> --
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> http://mailman.mit.edu/mailman/listinfo/piclist
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[RussellMc]

On 17 January 2018 at 22:54, John Gardner <***@gmail.com> wrote:

> I guess there's a reason data sheets are (usually) anonymous?
>

​Only if you want them to be. Often enough.

Genuine tale:
When looking at Asian data sheets
​(mostly but not only LEDs) ​
whose claims appeared suspect (and often the rest as well) I'd look for a
phrase that seemed liable to be unique to the data sheet concerned and
web-search for it. This quite often turned up a series of datasheets using
the same unlikely to be used phrase - often in data sheets for dissimilar
products (eg LEDs of a different power or colour) - and often from other
suppliers. Where it may have mattered it was often possible to trace the
data sheet :family" back to a parent from a (believed to be) known
reputable manufacturer.



Russell

​
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[s***@agilent.com]

Damned physics! ;o)

Time is the key here - you mention headroom for temporary overloads. What detects the overload? What interrupts the overload? How soon does it operate? What is the normal load?

Assuming that the normal load keeps the temperature rise within acceptable bounds, the time taken to interrupt the overload will determine the temperature rise you see in the resistor during the fault. If that time is indefinite, you need to deal with the excessive temperature through heatsinking or airflow or both. It is hard to get around P = I^2R. The thing the 5W buys you is some time. The greater heat capacity of the body, the greater surface area, etc. etc. means the time to reach critical temperature will be longer.

Given that this is for surviving fault conditions, I would be reluctant to add heatsinking just for that (COG, reliability, manufacturability etc.). Rather , add earlier detection and interruption of the fault. If critical, with some redundancy (COG ;o) ).

As an aside, while it does provide extra cooling to the resistor, the another (main?) reason for mounting power resistors off the PCB is to limit scorching of the board and possible delamination. This can happen well before solder melts.

Stephen


-----Original Message-----
From: piclist-***@mit.edu [mailto:piclist-***@mit.edu] On Behalf Of Bob Blick
Sent: Wednesday, 17 January 2018 5:28 PM
To: Microcontroller discussion list - Public. <***@mit.edu>
Subject: Re: [EE] Resistor temperature rise

A funny thing, though, is that I originally was using 2 watt resistors(metal oxide and also a different brand) and got these 5 watt wirewound ones hoping for a little cooler running. But comparing the graphs in their datasheets I noticed that, at 2 watts, the 5 watt and 2 watt resistors both showed very similar temperature rise. I would have thought the larger size of the 5 watt resistors would have had some effect. I have not measured the actual temperature rise though, who knows. Obviously the wirewound resistors can withstand a higher operating temperature than the metal oxide ones. Long term operation is not high power, I just wanted a little extra headroom for temporary overloads.

Cheerful regards,

Bob

________________________________________
From: piclist-***@mit.edu <piclist-***@mit.edu> on behalf of Sean Breheny
Sent: Tuesday, January 16, 2018 4:08 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] Resistor temperature rise

As you know, Bob, many component vendors do practice fuzzy specsmanship.
However, I think resistors are rather mild in this case. For example, it is standard practice for MOSFET vendors to rate their FETs according to the amount of current they could carry if you somehow magically kept the case at 25 deg C.This means that they only have to account for the junction to case thermal impedance as well as the heating in the leads of the package.
In real life you will NEVER get away with that much current for more than a split second.

As I understand it, for through-hole resistors, the rating is the max power the resistor can dissipate continuously, in free air (i.e., not mounted to a PCB) with the surrounding air at 25 deg C, for some specified lifetime, without changing its resistance value by more than X percent.

A good and generally accepted rule of thumb is that you can use a resistor at up to half its rated power, continuously, soldered to a PCB, provided that there are no other significant sources of heat in the vicinity of the resistor AND the air temperature in the enclosure (if there is one) is not much above 25 C even considering the dissipation from the resistor. This includes the PCB traces leading to the resistor - they must be large enough that they have negligible heating from the current passing through them. I had a board once where the (low value current sense) resistors lifted the traces off the board because I failed to take into account that the traces and the resistors would both generate heat. In other words, the resistor alone was within its specs and the traces were sized properly according to PCB trace width calculators but both of those ratings assume that their contribution is by far dominant but in this case, the heat from the resistor heated the copper jus!
t enough that it went into thermal runaway at the continuous operating current.



On Tue, Jan 16, 2018 at 4:17 PM, Bob Blick wrote:

> So basically they sell 5 watt resistors but if you solder them you
> can't push more than 2.5 watts out of them? I've looked at a lot of
> datasheets lately, and none of them make any mention at all about
> connection or mounting methods.
>
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[Bob Blick]

Hi Stephen,

I was hoping to buy a little time and prevent scorching and self-desoldering by using 5W instead of 2W resistors. But when I compared the spec sheets between the two resistors, at 2 watts the temperature rise is very similar. I don't really see how that can be, the 5W resistor is larger so I'd assume it would run cooler at the same power levels.

In some configurations there will be a thermal cutout on the power transistors, we'll see whether that activates before or after the resistors fail or do board damage.

Thanks,

Bob

________________________________________
From: piclist-***@mit.edu <piclist-***@mit.edu> on behalf of stephen.forrest
Sent: Monday, January 22, 2018 3:02 PM
To: ***@mit.edu
Subject: RE: [EE] Resistor temperature rise

Damned physics! ;o)

Time is the key here - you mention headroom for temporary overloads. What detects the overload? What interrupts the overload? How soon does it operate? What is the normal load?

Assuming that the normal load keeps the temperature rise within acceptable bounds, the time taken to interrupt the overload will determine the temperature rise you see in the resistor during the fault. If that time is indefinite, you need to deal with the excessive temperature through heatsinking or airflow or both. It is hard to get around P = I^2R. The thing the 5W buys you is some time. The greater heat capacity of the body, the greater surface area, etc. etc. means the time to reach critical temperature will be longer.

Given that this is for surviving fault conditions, I would be reluctant to add heatsinking just for that (COG, reliability, manufacturability etc.). Rather , add earlier detection and interruption of the fault. If critical, with some redundancy (COG ;o) ).

As an aside, while it does provide extra cooling to the resistor, the another (main?) reason for mounting power resistors off the PCB is to limit scorching of the board and possible delamination. This can happen well before solder melts.

Stephen

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[Allen Mulvey]

Most of you are probably aware of this but I had never seen
them before. Power resistors that stand on end and have feet
to keep them away from the board:
http://www.newark.com/w/c/passive-components/resistors-fixed
-value?range=inc-in-stock&st=sq&CMP=e-email-Buyer-230118-TE&
et_cid=29759719&et_rid=1338645273&cmp=

Allen

-----Original Message-----
From: piclist-***@mit.edu
[mailto:piclist-***@mit.edu] On Behalf Of Bob Blick
Sent: Tuesday, January 23, 2018 12:04 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] Resistor temperature rise

Hi Stephen,

I was hoping to buy a little time and prevent scorching and
self-desoldering by using 5W instead of 2W resistors. But
when I compared the spec sheets between the two resistors,
at 2 watts the temperature rise is very similar. I don't
really see how that can be, the 5W resistor is larger so I'd
assume it would run cooler at the same power levels.

In some configurations there will be a thermal cutout on the
power transistors, we'll see whether that activates before
or after the resistors fail or do board damage.

Thanks,

Bob

________________________________________
From: piclist-***@mit.edu <piclist-***@mit.edu> on
behalf of stephen.forrest
Sent: Monday, January 22, 2018 3:02 PM
To: ***@mit.edu
Subject: RE: [EE] Resistor temperature rise

Damned physics! ;o)

Time is the key here - you mention headroom for temporary
overloads. What detects the overload? What interrupts the
overload? How soon does it operate? What is the normal load?

Assuming that the normal load keeps the temperature rise
within acceptable bounds, the time taken to interrupt the
overload will determine the temperature rise you see in the
resistor during the fault. If that time is indefinite, you
need to deal with the excessive temperature through
heatsinking or airflow or both. It is hard to get around P =
I^2R. The thing the 5W buys you is some time. The greater
heat capacity of the body, the greater surface area, etc.
etc. means the time to reach critical temperature will be
longer.

Given that this is for surviving fault conditions, I would
be reluctant to add heatsinking just for that (COG,
reliability, manufacturability etc.). Rather , add earlier
detection and interruption of the fault. If critical, with
some redundancy (COG ;o) ).

As an aside, while it does provide extra cooling to the
resistor, the another (main?) reason for mounting power
resistors off the PCB is to limit scorching of the board and
possible delamination. This can happen well before solder
melts.

Stephen

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[Clint Jay]

This style of resistor:

https://www.jameco.com/z/212-7-2-Watt-6-8-Ohm-Ceramic-Wirewound-Resistor_2159752.html

can have all sorts of mounting hardware to dissipate heat (I've seen them
with fins), act as a board protecting stand off and even as a very simple
fusible link that needs a soldering iron to reset

On 23 Jan 2018 20:45, "Allen Mulvey" <***@amulvey.com> wrote:

Most of you are probably aware of this but I had never seen
them before. Power resistors that stand on end and have feet
to keep them away from the board:
http://www.newark.com/w/c/passive-components/resistors-fixed
-value?range=inc-in-stock&st=sq&CMP=e-email-Buyer-230118-TE&
et_cid=29759719&et_rid=1338645273&cmp=

Allen

-----Original Message-----
From: piclist-***@mit.edu
[mailto:piclist-***@mit.edu] On Behalf Of Bob Blick
Sent: Tuesday, January 23, 2018 12:04 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] Resistor temperature rise

Hi Stephen,

I was hoping to buy a little time and prevent scorching and
self-desoldering by using 5W instead of 2W resistors. But
when I compared the spec sheets between the two resistors,
at 2 watts the temperature rise is very similar. I don't
really see how that can be, the 5W resistor is larger so I'd
assume it would run cooler at the same power levels.

In some configurations there will be a thermal cutout on the
power transistors, we'll see whether that activates before
or after the resistors fail or do board damage.

Thanks,

Bob

________________________________________
From: piclist-***@mit.edu <piclist-***@mit.edu> on
behalf of stephen.forrest
Sent: Monday, January 22, 2018 3:02 PM
To: ***@mit.edu
Subject: RE: [EE] Resistor temperature rise

Damned physics! ;o)

Time is the key here - you mention headroom for temporary
overloads. What detects the overload? What interrupts the
overload? How soon does it operate? What is the normal load?

Assuming that the normal load keeps the temperature rise
within acceptable bounds, the time taken to interrupt the
overload will determine the temperature rise you see in the
resistor during the fault. If that time is indefinite, you
need to deal with the excessive temperature through
heatsinking or airflow or both. It is hard to get around P =
I^2R. The thing the 5W buys you is some time. The greater
heat capacity of the body, the greater surface area, etc.
etc. means the time to reach critical temperature will be
longer.

Given that this is for surviving fault conditions, I would
be reluctant to add heatsinking just for that (COG,
reliability, manufacturability etc.). Rather , add earlier
detection and interruption of the fault. If critical, with
some redundancy (COG ;o) ).

As an aside, while it does provide extra cooling to the
resistor, the another (main?) reason for mounting power
resistors off the PCB is to limit scorching of the board and
possible delamination. This can happen well before solder
melts.

Stephen

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[David C Brown]

High melting point solder?

On Tuesday, 16 January 2018, Bob Blick <***@outlook.com> wrote:

> What techniques are most effective in keeping power resistors from
> desoldering themselves from PC boards? I'm thinking there are basically
> three methods:
>
> 1. Don't use them at anywhere near their rated power. That's fine except
> if you need to tolerate the occasional overload condition.
>
> 2. Pour extra copper on and around the pads.
>
> 3. Use as much length of the wire leads as possible. That could really
> decrease the rigidity of the mounting.
>
> 4. ??
>
> Attached is a picture of the temperature chart. I'm using the 5 watt AC05.
> Note the 260 degree K rise at rated power.
>
> If I fold or clinch the leads it will keep the resistor from falling out,
> but the solder is going to melt. I imagine that won't give the greatest
> electrical connection :)
>
> Datasheet here:
>
> http://www.vishay.com/docs/28730/acac-at.pdf
>
> Thanks,
>
> Bob



--
__________________________________________
David C Brown
43 Bings Road
Whaley Bridge
High Peak Phone: [phone]
Derbyshire eMail: ***@gmail.com
SK23 7ND web: www.bings-knowle.co.uk/dcb
<http://www.jb.man.ac.uk/~dcb>



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[mbaum7901]

One approach is a mechanical rather than solder connection to the resistor
lead.
1. Tack or spot weld
2. Crimp
3. Screw terminal

The resistor wattage rating and corresponding temperature rise curves
describe what you can do with the resistor before the resistor fails (burst
into flames). The actual resistor wattage selection is then a function of
the system design.

The 5 watt resistor is really good for 5 watts if the system usage has the
resistor body and leads in contact with surfaces that can withstand the
resulting temperatures.

I am currently repairing a Ford F-250 overhead display console which has
1206 resistors that run so hot that they melt the solder and fall off the
board.

-----Original Message-----
From: piclist-***@mit.edu [mailto:piclist-***@mit.edu] On Behalf Of
David C Brown
Sent: Tuesday, January 16, 2018 15:54
To: Microcontroller discussion list - Public. <***@mit.edu>
Subject: Re: [EE] Resistor temperature rise

High melting point solder?

On Tuesday, 16 January 2018, Bob Blick <***@outlook.com> wrote:

> What techniques are most effective in keeping power resistors from
> desoldering themselves from PC boards? I'm thinking there are
> basically three methods:
>
> 1. Don't use them at anywhere near their rated power. That's fine
> except if you need to tolerate the occasional overload condition.
>
> 2. Pour extra copper on and around the pads.
>
> 3. Use as much length of the wire leads as possible. That could really
> decrease the rigidity of the mounting.
>
> 4. ??
>
> Attached is a picture of the temperature chart. I'm using the 5 watt AC05.
> Note the 260 degree K rise at rated power.
>
> If I fold or clinch the leads it will keep the resistor from falling
> out, but the solder is going to melt. I imagine that won't give the
> greatest electrical connection :)
>
> Datasheet here:
>
> http://www.vishay.com/docs/28730/acac-at.pdf
>
> Thanks,
>
> Bob



--
__________________________________________
David C Brown
43 Bings Road
Whaley Bridge
High Peak Phone: [phone]
Derbyshire eMail: ***@gmail.com
SK23 7ND web: www.bings-knowle.co.uk/dcb
<http://www.jb.man.ac.uk/~dcb>



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[Van Horn, David]

"I am currently repairing a Ford F-250 overhead display console which has
1206 resistors that run so hot that they melt the solder and fall off the board."


And these guys will design the computers which drive our cars......



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[John Gardner]

Found On Road Dead...
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[Allen Mulvey]

If I recall correctly there was some Korean War era stuff
that had spot welds.

Allen

-----Original Message-----
From: piclist-***@mit.edu
[mailto:piclist-***@mit.edu] On Behalf Of mbaum7901
Sent: Tuesday, January 16, 2018 5:18 PM
To: 'Microcontroller discussion list - Public.'
Subject: RE: [EE] Resistor temperature rise

One approach is a mechanical rather than solder connection
to the resistor
lead.
1. Tack or spot weld
2. Crimp
3. Screw terminal

The resistor wattage rating and corresponding temperature
rise curves
describe what you can do with the resistor before the
resistor fails (burst
into flames). The actual resistor wattage selection is then
a function of
the system design.

The 5 watt resistor is really good for 5 watts if the system
usage has the
resistor body and leads in contact with surfaces that can
withstand the
resulting temperatures.

I am currently repairing a Ford F-250 overhead display
console which has
1206 resistors that run so hot that they melt the solder and
fall off the
board.

-----Original Message-----
From: piclist-***@mit.edu
[mailto:piclist-***@mit.edu] On Behalf Of
David C Brown
Sent: Tuesday, January 16, 2018 15:54
To: Microcontroller discussion list - Public.
<***@mit.edu>
Subject: Re: [EE] Resistor temperature rise

High melting point solder?

On Tuesday, 16 January 2018, Bob Blick
<***@outlook.com> wrote:

> What techniques are most effective in keeping power
resistors from
> desoldering themselves from PC boards? I'm thinking there
are
> basically three methods:
>
> 1. Don't use them at anywhere near their rated power.
That's fine
> except if you need to tolerate the occasional overload
condition.
>
> 2. Pour extra copper on and around the pads.
>
> 3. Use as much length of the wire leads as possible. That
could really
> decrease the rigidity of the mounting.
>
> 4. ??
>
> Attached is a picture of the temperature chart. I'm using
the 5 watt AC05.
> Note the 260 degree K rise at rated power.
>
> If I fold or clinch the leads it will keep the resistor
from falling
> out, but the solder is going to melt. I imagine that won't
give the
> greatest electrical connection :)
>
> Datasheet here:
>
> http://www.vishay.com/docs/28730/acac-at.pdf
>
> Thanks,
>
> Bob



--
__________________________________________
David C Brown
43 Bings Road
Whaley Bridge
High Peak Phone: [phone]
Derbyshire eMail: ***@gmail.com
SK23 7ND web: www.bings-knowle.co.uk/dcb
<http://www.jb.man.ac.uk/~dcb>



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[mbaum7901]

I have seen the electrical connections to heating elements be spot welds.

-----Original Message-----
From: piclist-***@mit.edu [mailto:piclist-***@mit.edu] On Behalf Of
Allen Mulvey
Sent: Tuesday, January 16, 2018 17:08
To: 'Microcontroller discussion list - Public.' <***@mit.edu>
Subject: RE: [EE] Resistor temperature rise

If I recall correctly there was some Korean War era stuff that had spot
welds.

Allen

-----Original Message-----
From: piclist-***@mit.edu
[mailto:piclist-***@mit.edu] On Behalf Of mbaum7901
Sent: Tuesday, January 16, 2018 5:18 PM
To: 'Microcontroller discussion list - Public.'
Subject: RE: [EE] Resistor temperature rise

One approach is a mechanical rather than solder connection to the resistor
lead.
1. Tack or spot weld
2. Crimp
3. Screw terminal

The resistor wattage rating and corresponding temperature rise curves
describe what you can do with the resistor before the resistor fails (burst
into flames). The actual resistor wattage selection is then a function of
the system design.

The 5 watt resistor is really good for 5 watts if the system usage has the
resistor body and leads in contact with surfaces that can withstand the
resulting temperatures.

I am currently repairing a Ford F-250 overhead display console which has
1206 resistors that run so hot that they melt the solder and fall off the
board.

-----Original Message-----
From: piclist-***@mit.edu
[mailto:piclist-***@mit.edu] On Behalf Of David C Brown
Sent: Tuesday, January 16, 2018 15:54
To: Microcontroller discussion list - Public.
<***@mit.edu>
Subject: Re: [EE] Resistor temperature rise

High melting point solder?

On Tuesday, 16 January 2018, Bob Blick
<***@outlook.com> wrote:

> What techniques are most effective in keeping power
resistors from
> desoldering themselves from PC boards? I'm thinking there
are
> basically three methods:
>
> 1. Don't use them at anywhere near their rated power.
That's fine
> except if you need to tolerate the occasional overload
condition.
>
> 2. Pour extra copper on and around the pads.
>
> 3. Use as much length of the wire leads as possible. That
could really
> decrease the rigidity of the mounting.
>
> 4. ??
>
> Attached is a picture of the temperature chart. I'm using
the 5 watt AC05.
> Note the 260 degree K rise at rated power.
>
> If I fold or clinch the leads it will keep the resistor
from falling
> out, but the solder is going to melt. I imagine that won't
give the
> greatest electrical connection :)
>
> Datasheet here:
>
> http://www.vishay.com/docs/28730/acac-at.pdf
>
> Thanks,
>
> Bob



--
__________________________________________
David C Brown
43 Bings Road
Whaley Bridge
High Peak Phone: [phone]
Derbyshire eMail: ***@gmail.com
SK23 7ND web: www.bings-knowle.co.uk/dcb
<http://www.jb.man.ac.uk/~dcb>



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[mike brown]

Lots of solders available with much higher melting points. Any reason you
can't use those?

On Jan 16, 2018 17:31, "mbaum7901" <***@gmail.com> wrote:

> I have seen the electrical connections to heating elements be spot welds.
>
> -----Original Message-----
> From: piclist-***@mit.edu [mailto:piclist-***@mit.edu] On Behalf
> Of
> Allen Mulvey
> Sent: Tuesday, January 16, 2018 17:08
> To: 'Microcontroller discussion list - Public.' <***@mit.edu>
> Subject: RE: [EE] Resistor temperature rise
>
> If I recall correctly there was some Korean War era stuff that had spot
> welds.
>
> Allen
>
> -----Original Message-----
> From: piclist-***@mit.edu
> [mailto:piclist-***@mit.edu] On Behalf Of mbaum7901
> Sent: Tuesday, January 16, 2018 5:18 PM
> To: 'Microcontroller discussion list - Public.'
> Subject: RE: [EE] Resistor temperature rise
>
> One approach is a mechanical rather than solder connection to the resistor
> lead.
> 1. Tack or spot weld
> 2. Crimp
> 3. Screw terminal
>
> The resistor wattage rating and corresponding temperature rise curves
> describe what you can do with the resistor before the resistor fails (burst
> into flames). The actual resistor wattage selection is then a function of
> the system design.
>
> The 5 watt resistor is really good for 5 watts if the system usage has the
> resistor body and leads in contact with surfaces that can withstand the
> resulting temperatures.
>
> I am currently repairing a Ford F-250 overhead display console which has
> 1206 resistors that run so hot that they melt the solder and fall off the
> board.
>
> -----Original Message-----
> From: piclist-***@mit.edu
> [mailto:piclist-***@mit.edu] On Behalf Of David C Brown
> Sent: Tuesday, January 16, 2018 15:54
> To: Microcontroller discussion list - Public.
> <***@mit.edu>
> Subject: Re: [EE] Resistor temperature rise
>
> High melting point solder?
>
> On Tuesday, 16 January 2018, Bob Blick
> <***@outlook.com> wrote:
>
> > What techniques are most effective in keeping power
> resistors from
> > desoldering themselves from PC boards? I'm thinking there
> are
> > basically three methods:
> >
> > 1. Don't use them at anywhere near their rated power.
> That's fine
> > except if you need to tolerate the occasional overload
> condition.
> >
> > 2. Pour extra copper on and around the pads.
> >
> > 3. Use as much length of the wire leads as possible. That
> could really
> > decrease the rigidity of the mounting.
> >
> > 4. ??
> >
> > Attached is a picture of the temperature chart. I'm using
> the 5 watt AC05.
> > Note the 260 degree K rise at rated power.
> >
> > If I fold or clinch the leads it will keep the resistor
> from falling
> > out, but the solder is going to melt. I imagine that won't
> give the
> > greatest electrical connection :)
> >
> > Datasheet here:
> >
> > http://www.vishay.com/docs/28730/acac-at.pdf
> >
> > Thanks,
> >
> > Bob
>
>
>
> --
> __________________________________________
> David C Brown
> 43 Bings Road
> Whaley Bridge
> High Peak Phone: [phone]
> Derbyshire eMail: ***@gmail.com
> SK23 7ND web: www.bings-knowle.co.uk/dcb
> <http://www.jb.man.ac.uk/~dcb>
>
>
>
> *Sent from my etch-a-sketch*
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[Peter]

Back in the late 80's to early 90's when just starting out in
engineering at a tv station. My mentor, had this issue on many power
supplies, they designed and built, back then. All their engineering was
custom and done in house. He and his other colleagues solved this
plaguing issue in a very unconventional way, but very reliable way,
which I remember well.

1. Soldered brass tubes to the legs, inserting the ceramic resistor leg
into the tube and filling with solder;
2. Tube diameter, was chosen, based on previous experimenting, not sure
now, maybe 3mm to 6mm;
3. Extended the legs if it was required;
4. Moulded the resistor in either cement, concrete or other ceramics
(unsure now), in a 2 or 4 litre ice cream container size (about
that, sometimes bigger), with the brass tubes now the resistor legs,
which then all mounted to the custom part of the PCB;
5. Sometimes, they moulded a coiled brass or copper pipe inside, and
pumped water through as well;

These systems worked commercially day and night for many years in the
stations environment, were never turned off, they just worked.

Because space was not an issue, they needed reliability. I even think
they reduced their breakdowns to nil for that particular section.

So may not be able to be implemented, but thought I'd share anyway.

Was very interesting!

Peter

------------------------------------------------------------------------
On 17/01/2018 6:15 AM, Bob Blick wrote:
> What techniques are most effective in keeping power resistors from desoldering themselves from PC boards? I'm thinking there are basically three methods:
>
> 1. Don't use them at anywhere near their rated power. That's fine except if you need to tolerate the occasional overload condition.
>
> 2. Pour extra copper on and around the pads.
>
> 3. Use as much length of the wire leads as possible. That could really decrease the rigidity of the mounting.
>
> 4. ??
>
> Attached is a picture of the temperature chart. I'm using the 5 watt AC05. Note the 260 degree K rise at rated power.
>
> If I fold or clinch the leads it will keep the resistor from falling out, but the solder is going to melt. I imagine that won't give the greatest electrical connection :)
>
> Datasheet here:
>
> http://www.vishay.com/docs/28730/acac-at.pdf
>
> Thanks,
>
> Bob

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[RussellMc]

In prototyping use I have added soldered on brass shim heat-sink flags to
power diodes and to power resistors.
Worked OK in that context.

In your context - if you run the resistor so that it is withing spec for
desired lifetime at operating temperature most of the heta loss must be by
radiation, with the leads then largely providing mechanical and electrical
connection.
The object is then to have leads which provide adequate mechanical support
and which LIMIT heat transmission to the board. Lengthening the leads AND
heat flagging them in some manner at tghe PCB end and not all the way up to
the resistor may [tm] act similarly to a simple RC filter with the lead
limiting energy flow rate and the flag having to deal with the energy that
does pass down the lead. This increases the temperature of the resistor
above what it would otherwise be at and reduces board temperature.

An 'adequate' result may be achievable by each coiling the leads in a tight
spiral so that thermal conductivity is much reduced. This could perhaps be
done aroung an insulatorv which is left in place to add mechanical
robustness. (Something like Richard's second drawing but with many small
turns. Electrical resistance rises slightly due to longer leads. Inductance
may be an issue depending on application.


Russell
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[John Ferrell]

Silver Solder or brazing?


On 1/16/2018 2:15 PM, Bob Blick wrote:
> What techniques are most effective in keeping power resistors from desoldering themselves from PC boards? I'm thinking there are basically three methods:

--
John Ferrell W8CCW
Julian NC 27283
It is better to walk alone,
than with a crowd going the wrong direction.
--Diane Grant


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[Forrest Christian (List Account)]

Since this thread seems to be still alive, I'll add my $0.02:

The degrees rise figure is the typically body temperature, not the
temperature of the leads at the board, which be much cooler. Note that
on page 10 of the datasheet you linked it shows recommended lead
termination bending, effectively offseting the resistor off of the board by
8mm. I was trying to find a good thermal camera picture to show this, the
closest was at https://www.youtube.com/watch?v=-e-qIjttIIY&t=134s , at 1:57
in, there is a thermal camera pic which shows the leads basically being
cold, while the resistor body itself being hot. I wouldn't be surprised
to find that when mounted as recommended, the units will not desolder from
the board.

Personally I try to run things as cool as possible, if I was engineering
for 5W dissipation, I would use at minimum a 7.5W if not a 10W resistor.
Unless I was *very* sure of the engineering.

Also, be mindful that the rating is at 40*C ambient *around the resistor*.
If you don't provide adequate airflow around the resistor to eliminate a
thermal air hotspot around the resistor, you're actually going to have an
even lower rating.



On Tue, Jan 16, 2018 at 12:15 PM, Bob Blick <***@outlook.com> wrote:

> What techniques are most effective in keeping power resistors from
> desoldering themselves from PC boards? I'm thinking there are basically
> three methods:
>
> 1. Don't use them at anywhere near their rated power. That's fine except
> if you need to tolerate the occasional overload condition.
>
> 2. Pour extra copper on and around the pads.
>
> 3. Use as much length of the wire leads as possible. That could really
> decrease the rigidity of the mounting.
>
> 4. ??
>
> Attached is a picture of the temperature chart. I'm using the 5 watt AC05.
> Note the 260 degree K rise at rated power.
>
> If I fold or clinch the leads it will keep the resistor from falling out,
> but the solder is going to melt. I imagine that won't give the greatest
> electrical connection :)
>
> Datasheet here:
>
> http://www.vishay.com/docs/28730/acac-at.pdf
>
> Thanks,
>
> Bob
> --
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>
>


--
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***@imach.com | http://www.packetflux.com
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[Van Horn, David]

Search the archives for instances of "Low voltage the incompetent ignition source" with regard to some fun I had years ago with a failed H-Bridge that couldn't get rid of heat fast enough.
The 1W average input power from the switcher (in hiccup mode) was enough to take the board from failed to flaming and to keep it there.

Redesign of the H bridge solved the issue, and relayout of the PCB with lots of copper pour assured that even if the new design failed, the heat dissipation was good enough to keep the temperatures down.

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[Sean Breheny]

Nice demo video but please be aware that thermal images of unpainted
metallic parts can be misleading. A perfect mirror-like surface would
actually show the temperature of the surroundings, not its own temperature.
A real typical clean metal surface will show a weighted average of its
surroundings and its own temperature, with the contribution from its own
temperature being about 30% of what a perfect blackbody would radiate at
the same temperature. A tarnished or oxidized metal surface can have an
emissivity which is much closer to ideal, typically 60 to 80%. However, it
is best to always use an organic material when measuring temperature
thermographically because almost all organic materials are within 10% of a
true blackbody so you don't have to worry about calibrating the camera for
your particular surface.

To make a long story short, the leads of a resistor will usually appear
cooler than they really are in a thermal image.

On Tue, Jan 23, 2018 at 4:55 PM, Forrest Christian (List Account) <
***@packetflux.com> wrote:

> Since this thread seems to be still alive, I'll add my $0.02:
>
> The degrees rise figure is the typically body temperature, not the
> temperature of the leads at the board, which be much cooler. Note that
> on page 10 of the datasheet you linked it shows recommended lead
> termination bending, effectively offseting the resistor off of the board by
> 8mm. I was trying to find a good thermal camera picture to show this, the
> closest was at https://www.youtube.com/watch?v=-e-qIjttIIY&t=134s , at
> 1:57
> in, there is a thermal camera pic which shows the leads basically being
> cold, while the resistor body itself being hot. I wouldn't be surprised
> to find that when mounted as recommended, the units will not desolder from
> the board.
>
> Personally I try to run things as cool as possible, if I was engineering
> for 5W dissipation, I would use at minimum a 7.5W if not a 10W resistor.
> Unless I was *very* sure of the engineering.
>
> Also, be mindful that the rating is at 40*C ambient *around the resistor*.
> If you don't provide adequate airflow around the resistor to eliminate a
> thermal air hotspot around the resistor, you're actually going to have an
> even lower rating.
>
>
>
>
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[Forrest Christian (List Account)]

I agree with your assessment of this video, as far as the accuracy of the
relative temperatures of the wires. However, it's still true that the
center of the resistor is likely to be much hotter than the leads.
Whenever I've had to deal with something like this, I've ended up
attaching a thermocouple (or thermocouples) at the relevant points to
directly measure the temperatures at the interesting points to validate
expectations.

I really wish I could find the excellent appnote or discussion in a
textbook somewhere that I specifically remembered discussing the whole
thermal environment around leaded power resistors, and how to deal with
these issues, including how to predict the temperature at the leads.

On Tue, Jan 23, 2018 at 4:53 PM, Sean Breheny <***@cornell.edu> wrote:

> Nice demo video but please be aware that thermal images of unpainted
> metallic parts can be misleading. A perfect mirror-like surface would
> actually show the temperature of the surroundings, not its own temperature.
> A real typical clean metal surface will show a weighted average of its
> surroundings and its own temperature, with the contribution from its own
> temperature being about 30% of what a perfect blackbody would radiate at
> the same temperature. A tarnished or oxidized metal surface can have an
> emissivity which is much closer to ideal, typically 60 to 80%. However, it
> is best to always use an organic material when measuring temperature
> thermographically because almost all organic materials are within 10% of a
> true blackbody so you don't have to worry about calibrating the camera for
> your particular surface.
>
> To make a long story short, the leads of a resistor will usually appear
> cooler than they really are in a thermal image.
>
> On Tue, Jan 23, 2018 at 4:55 PM, Forrest Christian (List Account) <
> ***@packetflux.com> wrote:
>
> > Since this thread seems to be still alive, I'll add my $0.02:
> >
> > The degrees rise figure is the typically body temperature, not the
> > temperature of the leads at the board, which be much cooler. Note
> that
> > on page 10 of the datasheet you linked it shows recommended lead
> > termination bending, effectively offseting the resistor off of the board
> by
> > 8mm. I was trying to find a good thermal camera picture to show this,
> the
> > closest was at https://www.youtube.com/watch?v=-e-qIjttIIY&t=134s , at
> > 1:57
> > in, there is a thermal camera pic which shows the leads basically being
> > cold, while the resistor body itself being hot. I wouldn't be surprised
> > to find that when mounted as recommended, the units will not desolder
> from
> > the board.
> >
> > Personally I try to run things as cool as possible, if I was engineering
> > for 5W dissipation, I would use at minimum a 7.5W if not a 10W resistor.
> > Unless I was *very* sure of the engineering.
> >
> > Also, be mindful that the rating is at 40*C ambient *around the
> resistor*.
> > If you don't provide adequate airflow around the resistor to eliminate a
> > thermal air hotspot around the resistor, you're actually going to have an
> > even lower rating.
> >
> >
> >
> >
> --
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>



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Tel: [phone] | Address: 3577 Countryside Road, Helena, MT 59602
***@imach.com | http://www.packetflux.com
<http://www.linkedin.com/in/fwchristian> <http://facebook.com/packetflux>
<http://twitter.com/@packetflux>
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[Bob Blick]

Hi Forrest,
Good points, and thanks for the video. I decided to test for myself, so I made a string of 2W metal oxide resistors connected to a power supply and cranked it up so the resistors were at 2W each. With full leads, 8mm from any surface and 21C ambient.

Using .4mm(0.15") 63/37 solder for my test, solder melted quickly on the body of the resistor. But I could not get solder to melt on the leads at all. Attached is a picture with a small piece of solder wrapped tightly about 1mm from the resistor body. After 20 minutes there wasn't even any flux escaping from the ends of the solder.

A magnet sticks to the leads of these resistors. Most power resistors seem to have similar leads. I have another (more expensive) brand that are non-magnetic, and although their datasheet shows the leads as "CuFe" they are not more specific and I did not do a solder test on them.

In summary, this makes me feel good. I wanted to rule out failures due to resistors desoldering and falling out of the board. Using lead-free solder will give me an extra 10-15 degrees of margin for high ambient conditions. Board scorching is another risk, so I will space them off the board as much as reasonable.

As I've mentioned before, I'm only worried about a temporary overload condition until other safety systems kick in. Under normal conditions the resistors will be dissipating under 1W.

Thanks to everyone for your input, this has been fun.

Bob

________________________________________
From: piclist-***@mit.edu <piclist-***@mit.edu> on behalf of Forrest Christian
Sent: Tuesday, January 23, 2018 1:55 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] Resistor temperature rise

Since this thread seems to be still alive, I'll add my $0.02:

The degrees rise figure is the typically body temperature, not the
temperature of the leads at the board, which be much cooler. Note that
on page 10 of the datasheet you linked it shows recommended lead
termination bending, effectively offseting the resistor off of the board by
8mm. I was trying to find a good thermal camera picture to show this, the
closest was at https://www.youtube.com/watch?v=-e-qIjttIIY&t=134s , at 1:57
in, there is a thermal camera pic which shows the leads basically being
cold, while the resistor body itself being hot. I wouldn't be surprised
to find that when mounted as recommended, the units will not desolder from
the board.

[Forrest Christian (List Account)]

If you want to be triple safe, do some of the additional things you
suggested in your original post. Probably not necessary, but I always
tend to overengineer anything which takes any amount of heat (having had
components fall of boards myself, although usually only surface mount fets
and/or regulators in fault conditions). The biggest thing I've discovered
is to put as much outer-layer copper as possible at the component leads, as
this will help pull heat away from the joint as well.

But I'm glad to hear that theory and past experience worked in this case ;)


On Thu, Jan 25, 2018 at 10:48 AM, Bob Blick <***@outlook.com> wrote:

> Hi Forrest,
> Good points, and thanks for the video. I decided to test for myself, so I
> made a string of 2W metal oxide resistors connected to a power supply and
> cranked it up so the resistors were at 2W each. With full leads, 8mm from
> any surface and 21C ambient.
>
> Using .4mm(0.15") 63/37 solder for my test, solder melted quickly on the
> body of the resistor. But I could not get solder to melt on the leads at
> all. Attached is a picture with a small piece of solder wrapped tightly
> about 1mm from the resistor body. After 20 minutes there wasn't even any
> flux escaping from the ends of the solder.
>
> A magnet sticks to the leads of these resistors. Most power resistors seem
> to have similar leads. I have another (more expensive) brand that are
> non-magnetic, and although their datasheet shows the leads as "CuFe" they
> are not more specific and I did not do a solder test on them.
>
> In summary, this makes me feel good. I wanted to rule out failures due to
> resistors desoldering and falling out of the board. Using lead-free solder
> will give me an extra 10-15 degrees of margin for high ambient conditions.
> Board scorching is another risk, so I will space them off the board as much
> as reasonable.
>
> As I've mentioned before, I'm only worried about a temporary overload
> condition until other safety systems kick in. Under normal conditions the
> resistors will be dissipating under 1W.
>
> Thanks to everyone for your input, this has been fun.
>
> Bob
>
> ________________________________________
> From: piclist-***@mit.edu <piclist-***@mit.edu> on behalf of
> Forrest Christian
> Sent: Tuesday, January 23, 2018 1:55 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] Resistor temperature rise
>
> Since this thread seems to be still alive, I'll add my $0.02:
>
> The degrees rise figure is the typically body temperature, not the
> temperature of the leads at the board, which be much cooler. Note that
> on page 10 of the datasheet you linked it shows recommended lead
> termination bending, effectively offseting the resistor off of the board by
> 8mm. I was trying to find a good thermal camera picture to show this, the
> closest was at https://www.youtube.com/watch?v=-e-qIjttIIY&t=134s , at
> 1:57
> in, there is a thermal camera pic which shows the leads basically being
> cold, while the resistor body itself being hot. I wouldn't be surprised
> to find that when mounted as recommended, the units will not desolder from
> the board.
>
> --
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>
>


--
*Forrest Christian* *CEO**, PacketFlux Technologies, Inc.*
Tel: [phone] | Address: 3577 Countryside Road, Helena, MT 59602
***@imach.com | http://www.packetflux.com
<http://www.linkedin.com/in/fwchristian> <http://facebook.com/packetflux>
<http://twitter.com/@packetflux>
--
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist